Most forms of biological processes for treatment of wastewater involve introducing oxygen-containing gas into wastewater with some form of energy-consuming apparatus. Generally, an electric motor is the energy consumer, and it powers some kind of agitator, compressor or blower that provides driving force to distribute the oxygen-containing gas in one or more tanks containing wastewater. For many years it has been apparent that the cost of electricity to operate such equipment is one of the largest, and often the largest, operational cost of wastewater treatment plants.
In the early history of the art of biological treatment, process control was “manual”. Aided to an inadequate extent by visual observation and by instrumentation that was usually limited and rudimental, plant operating personnel adjusted gas flow in an attempt to match that flow to the amount of oxygen consumed in the biological process. Too much flow, overshoot, wasted electricity. Too little, undershoot, impaired the quality of treatment.
As the art progressed, it was recognized that savings in electricity and more consistent quality of treatment could be achieved with better and more complete instrumentation. Then, it began to be apparent that major gains in energy savings and quality could be attained through automatic control of gas flows and other aspects of the processes.
Since at least as early as the 1960s, efforts at automated control of the flow of oxygen-containing gas into biological wastewater treatment processes have included measurements of the DO (dissolved oxygen) level in the wastewater in the treatment tank. Gas flow is automatically reduced if DO exceeds a predetermined target or set point and increased if DO falls below the target. This mode of operation reduced but did not eliminate the problem of overshoot and undershoot of the true oxygen and energy requirements of the biological processes.
Since as least as early as the 1970s, the need to conserve energy and tightening regulations on plant effluent quality have provided ample and continuing motivation to develop better forms of automated control. However, despite many suggestions for additional and/or other modes of automatic control, in actual practice, control based primarily on DO levels, with ensuing energy wastage and quality challenges, has remained quite popular.
Present-day continuation of the popularity of control based primarily on DO measurements, accompanied by wastefulness and quality problems, suggests there is a long-felt, unsatisfied need for improvements in control of biological processes for the treatment of wastewater. The present invention seeks to fulfil this need.